This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Aldo-Keto Reductase (AKR) 1D1 (steroid 5-beta-reductase) catalyzes the conversion of all Delta(4)-3-ketosteroids to form 5-beta-dihydrosteroids, a first step in the clearance of steroid hormones, and an essential step in the synthesis of all bile-acids. The reaction is unique in steroid enzymology since hydride transfer from NADPH yields a A/B-ring cis-configuration producing a 90 degree bend in steroid structure. Recently we determined the X-ray crystal structure of human Delta(4)-3-ketosteroid 5-beta-reductase (AKR1D1) complexed with NADP+ and testosterone at 1.62 [unreadable] resolution. This structure represents the first crystal structure of a mammalian steroid hormone carbon-carbon double bond reductase. However, the structure revealed that testosterone binds in an unproductive mode. We are looking to determine the structure of AKR1D1 in complex with several substrates that would present